The present invention relates to the field of ultrasonic inspection technology.
As known in the related art (e.g. see JP-A-2001-228128), there is an instrument in which two ultrasonic probes, that is, a transmitter probe and a receiver probe are fixedly disposed at a fixed distance and a crack inside a subject to be inspected is ultrasonically inspected using a TOFD (Time of Flight Diffraction) technique; the TOFD technique is defined in British Standard BS7706 (1993); and according to the TOFD technique, the incident angle with which an ultrasonic wave transmitted from the transmitter probe enters the subject to be inspected is set to be in a range of from 45 degrees to 55 degrees while the receiving angle with which a diffraction echo derived from the ultrasonic wave and coming from a tip of a crack in the subject to be inspected is also set to be in a range of from 45 degrees to 55 degrees.
However, there is a fear that the intensity of the diffraction echo is lowered due to the divergence of the transmitted ultrasonic beam so that the performance of crack detection deteriorates. As a technique for wiping out such a fear, known are JP-A-2001-228128 (see pages 2-4 and FIGS. 1-8) and JP-A-2001-228126 (see pages 4-5 and FIGS. 1-4) in which an ultrasonic beam transmitted from a transmitter probe is converged to irradiate the tip of a crack therewith, and the diffraction echo detection area of a receiver probe is also converged to detect the diffraction echo efficiently.
In order to detect the diffraction echo efficiently, it is known that it is preferable in view of efficiency that the incident angle of the ultrasonic wave is 45 degrees, as disclosed in JP-A-2001-228126 (see pages 4-5 and FIG. 2).
Further, JP-A-2002-62281 (see pages 3-6 and FIGS. 1-12) discloses a technique in which an element set having a plurality of elements assembled in the form of a transmitter element array and a receiver element array to be mounted in one and the same casing is used while a delay circuit is connected to each element of the element set so as to gradually shift the timing for the delay circuit to excite the element. Thus, the angle of refraction of ultrasonic waves is finely adjusted through electronic scan while the traveling direction of the ultrasonic waves is controlled. In such a manner, the depth of a crack is measured.
In these conventional examples, ultrasonic inspection is carried out in the TOFD (Time of Flight Diffraction) technique in which two ultrasonic probes, that is, a transmitter probe and a receiver probe are fixed at a fixed distance, and further both the incident angle and the receiving angle of ultrasonic waves are set to be in a range of from 45 degrees to 55 degrees. The reason why those angles are regulated to be 45-55 degrees is as follows. That is, it has been heretofore considered that a diffraction echo obtained by irradiation of a tip of a crack with an ultrasonic beam is intensive in the direction of 45-55 degrees. Thus, such a range has been generally adopted.
However, in the TOFD technique in which both the incident angle and the receiving angle of ultrasonic waves are set to be about 45-55 degrees, the ultrasonic transmitter probe and the ultrasonic receiver probe are fixedly disposed at a fixed distance wide enough to retain the transmitting and receiving angles in the range of from about 45 degrees to about 55 degrees. This results in increase of the total external dimensions of the ultrasonic transmitter probe and the ultrasonic receiver probe. Thus, there is a problem that this technique cannot be applied to ultrasonic inspection of a place to be inspected or a narrow portion small in foot print of each transmitter/receiver probe. In addition, there is a problem that the course of the ultrasonic waves from the transmitter side to the receiver side is so long that the intensity of the received ultrasonic waves becomes weak.